(i) Field of the Invention
This invention relates to the stages for the preparation or treatment of metal surfaces taking place in the course of processes for the production or processing of flat products (such as sheet metal) or hollow bodies (bottles, valves, etc.).
Depending on the part of the production line for these products at which these stages take place, these preparation stages provide for cleaning, degreasing or activation of the surface, often prior to a subsequent process, which may be, e.g. annealing, the electroplating of zinc, aluminium, tin or alloys thereof, or the deposition of organic coatings, such as varnishes or paints, or inorganic coatings, such as nitrides or films based on silicon.
A stage of this kind for the preparation of surfaces may also take place at the end of the production line, e.g. just before sheet metal of this kind is coiled.
The products are made of steel, stainless steel, copper or, e.g. aluminium, depending on the final destination of the products in question (e.g. motor vehicles, electric household appliances, buildings or packaging, such as tins).
(ii) Description of Related Art
The methods most commonly used to effect these operations for the preparation of surfaces include first and foremost liquid-phase methods.
Examples in this category include the case of a preliminary degreasing stage effected with the aid of solvents, usually containing chlorine or fluorine (at present subject to very strict regulations) or methods consisting in degreasing the metal surface in a chemical bath in an aqueous medium containing acid, alkaline or neutral products, these having disadvantages associated with the required subsequent treatment of the waste water after cleaning.
In this context, a second category of methods for the treatment or preparation of metal surfaces has emerged, these being referred to as "dry methods". One example in this category is cleaning surfaces by gaseous plasma under low pressure using an electric discharge. This method is at first sight very attractive, as it has the advantage of showing great respect for the environment. It unites two separate operations of the electric discharge, i.e. on the one hand, the intense ion bombardment induced under certain conditions by the ionized medium, i.e. a cleaning operation by atomization, and, on the other hand, the method makes use of the high reactivity of the excited atoms and molecules present in the plasma in order to form volatile compounds with the bodies to be eliminated present on the surface of the sheet metal, i.e. chemical action on bodies of an essentially organic nature. The gaseous atmosphere generally preferred for this method of pickling under electric discharge contains a high percentage of argon on account of its highly plasmagenic gas quality and on account of the high mass of its ions promoting the atomization of this gas. It is moreover enriched with oxygen or hydrogen in order to use part of the chemical action of the discharge by eliminating the organic bodies present on the surface of the sheet metal by the formation of light compounds of the CO.sub.x or CH.sub.y type.
The high potential of this process cannot hide its major disadvantages connected to the fact that it is carried out under low pressure, or even in vacuo. These pressure conditions are undeniably curbing its development as they are relatively incompatible with the treatment of large metal surfaces or with high production rates (as is the case with the industrial production of sheet metal).
The use of low pressures of this kind moreover represents considerable additional expenditure. In a more general context, in French Patent Application published under the No. 2 697 456, the Applicant proposed a process for the plasma fluxing of metal surfaces prior to soldering or tinning at atmospheric pressure, the plasma being created by means of a microwave source or a corona discharge transferred via ports arranged in a suitable manner in a dielectric layer situated above the component to be treated.
Although this application provides an advantageous solution to the problem of the fluxing of metal surfaces, the Applicant has demonstrated the fact that the proposed process could be improved, inter alia with respect to:
its yield (ratio between the power introduced in order to create the plasma and the density of the species produced actually interacting with the support to be treated) or the energy density obtained (in the case of corona discharge it only reaches a few W per dielectric unit area), which, if increased, could allow for shorter treatment times, PA0 but also as a result of limiting "geometric" factors: in the case of conventional corona discharge, the distance between the electrode and the sample is quite critical and must be kept very small, which can cause problems in the case of uneven substrates. In the case of microwave discharge, it gives rise to the formation of a plasmagenic spot of predetermined dimensions limited by the plasma source, PA0 moreover, a plasma such as the one created in this document by definition contains ionic species and electrons (and therefore electrically charged species) which can induce ion bombardment and thus atomization of the metal surface which can in some cases prove to be harmful (e.g. when the metal surface is covered in a coating it is desired not to alter). PA0 to operate substantially at atmospheric pressure, PA0 to obtain high flexibility with respect to the distance between the object to be treated and the device used to carry out this treatment, PA0 to avoid contact between the components and electrically charged species, and PA0 to offer improved energy density allowing for an increased treatment rate.
In parallel to this, in French Patent Application published under the No. 2 692 730, the contents of which are incorporated hereby reference, the Applicant proposed a device for the production of excited or unstable gas molecules operating substantially at atmospheric pressure.